Vocabulary byte - The technical term for 8 bits of data. The standard fundamental unit (or “chunk size”) underlying most computing systems today measured in “megabytes”, “kilobytes”, “gigabytes”, etc.

CS Principles U2L2 Text Compression

U2L2 Text Compression Objectives SWBAT: Collaborate with a peer to find a solution to a text compression problem using the Text Compression Widget (lossless compression scheme). Explain why the optimal amount of compression is impossible or “hard” to identify. Explain some factors that make compression challenging. Develop a strategy (heuristic algorithm) for compressing text. Describe the purpose and rationale for lossless compression.

U2L2 Vocab Heuristic - a problem solving approach (algorithm) to find a satisfactory solution where finding an optimal or exact solution is impractical or impossible. Lossless Compression - a data compression algorithm that allows the original data to be perfectly reconstructed from the compressed data.

Text Compression At some point we reach a physical limit of how fast we can send bits and if we want to send a large amount of information faster, we have to find a way to represent the same information with fewer bits - we must compress the data. The basic principle behind compression is to develop a method or protocol for using fewer bits to represent the original information. The way we represent compressed data in this lesson, with a “dictionary” of repeated patterns is similar to the LZW compression scheme (Lempel-Zig-Welch) LZW is used for text (zip files) with the GIF image file format.

Optimal Compression? There is no single correct way to compress text using the method we use in this lesson because a) there is no known algorithm for finding an optimal solution, and b) we don’t even know a way to verify whether a given solution is optimal. There is no way to prove it or derive it beyond trying all possibilities by brute force. This is an example of an algorithm that cannot run in a “reasonable amount of time” - one of the CSP learning objectives.

Connections to Programming Recognizing patterns that repeat, then representing those patterns as abstract symbols, then further recognize patterns within those patterns, This is very similar to the kinds of abstractions we develop when writing functions and procedures when programming.

U2L2 Prompt: Prompt: (respond to both prompts) “When you send text messages to a friend, do you spell every word correctly?” Do you use abbreviations for common words? List as many as you can. Write some examples of things you might see in a text message that are not proper English. “Why do you use these abbreviations? What is the benefit?”

Text Compression Today’s class is about compression When you abbreviate or use coded language to shorten the original text, you are “compressing text.” Computers do this too, in order to save time and space. The art and science of compression is about figuring out how to represent the SAME DATA with FEWER BITS. Why is this important? One reason is that storage space is limited and you’d always prefer to use fewer bits if you could. A much more compelling reason is that there is an upper limit to how fast bits can be transmitted over the Internet. What if we need to send a large amount of text faster over the Internet, but we’ve reached the physical limit of how fast we can send bits? Our only choice is to somehow capture the same information with fewer bits; we call this compression.

Activity (Classwork) Complete the U2L2 Activity Guide – Decode This Message